1. Field of the Invention
The invention relates to the passive processing of audio signals from a plurality of sources having substantially different signal levels, and for allowing selective mixing of the signals for subsequent amplification.
2. The Prior Art
Devices for selective mixing of audio signals from a plurality of sources are well known in the art. Such mixers may be divided generally into two types. A first type is the active mixer which has amplification and equalization circuitry for accepting signals from sources having differing output characteristics, in particular having different output signal levels and output impedances. Such mixing devices have active elements which serve to generally equalize the signal levels and output impedances prior to mixing, so that the signals to be mixed are of substantially comparable characteristics and may be provided to a single output amplifier. One such active mixer is the "DISC/MIC MIXER" manufactured by Lafayette Radio Corporation of New York and sold under stock No. 99-46955W. Another such mixer having active elements is Type MA-62, manufactured by Pioneer Radio Corporation of Japan.
A mixing device having active elements is further shown in the audio signal programming apparatus of U.S. Pat. No. 2,995,630, wherein a preamplifier is provided between a microphone and a mixing switch arrangement, while no such preamplifier is provided between a phonograph turntable and the mixing switch arrangement.
The second general type of mixing system requires that signals of substantially equal characteristics be supplied thereto from the respective audio signal sources in order to avoid deterioration in output quality. Such systems are shown, for example, in U.S. Pat. Nos. 3,809,812 and 2,110,358. Further devices, such as those shown in U.S. Pat. Nos. 2,530,251 and 2,530,252, have a plurality of sources of differing signal characteristics, but provide one or more preamplifiers for equalizing the signal characteristics.
Aside from the mixing devices, amplifier units are known in the art which have tape monitoring functions. Such amplifier units comprise generally a plurality of input terminals for receiving various input source signals along with one or more preamplifiers for equalizing some of the input signals before supplying such signals to an output audio amplifier stage. Many such amplifier units have an arrangement of switches (or a single rotary switch) whereby a low level signal input, such as a phono or microphone input, is coupled to a preamplifier stage. The preamplifier stage output is in turn coupled to a tape monitor output. At the same time, a tape input terminal of the amplifier unit may be selectively coupled to the input of the audio output amplifier via a tape monitor switch.
Such an arrangement is conventionally used for monitoring the tape recording of a signal from a low-level signal source. For example, a phono pickup is coupled to the phono input and preamplified within the amplifier unit, the output of the preamplifier stage being supplied via the tape monitor output to the record head of a tape recorder. At the same time, the tape recorder output (such as from a record monitor head on the tape recorder) is coupled to the tape recorder input of the amplifier unit and supplied via the tape monitor switch and the audio output amplifier to a headphone or speaker. In this manner, the operator can monitor the signal transferred from the phonograph to the tape recorder with the phonograph output signal being preamplified prior to recording.
This use of the tape monitor feature of a conventional amplifier has been used to further advantage by the coupling of an active equalizer between the amplifier unit and the tape recorder as taught in U.S. Pat. No. 3,914,549. However, none of the aforementioned prior art arrangements teaches a passive mixing apparatus which makes use of the tape monitor feature of conventional amplifier or receiver units to permit mixing of program material from a plurality of sources having dissimilar signal characteristics.